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Review Comments Probable Maximum Precipitation (PMP) Site-Specific Study for Cherry Creek Reservoir (Final Report - Draft) April 2003 RFP PDA-0133W-Cherry Creek Review Comments provided by: Louis C. Schreiner - Technical Review Panel Member As mentioned in the final report and summarized at a pre-final meeting held April 2, 2003, report authors provided their view as to the main reasons why Applied Weather Associates (A W A) estimates ofPMP were lower by as much as 34 percent for 24 hours and 38 percent for 72 hours than those developed by the National Weather Service (NWS) for the Cherry Creek drainage. From the packet (Power Point presentation) provided at the pre-final meeting, the A W A team lists three primary reasons why PMP from the 1995 NWS study should be further reduced. These are: I. Moisture depletion by the Palmer and Kiowa barriers. The A W A team states that in adjusting the 1995 NWS study for Cherry Creek this affect causes a 10 percent reduction. I like to point out that in chapter 3 of Hydrometeorological Report 55A (HMR 55A) that .barrier adjustments were applied in the development of the F AFP analysis. I do not believe that additional reduction is warranted; it would only act as a suggested factor applied twice. 2. Spatial distribution of extreme rainfall. The A W A team indicates that an additional reduction to the PMP from the 1995 NWS study of 21 percent can be realized if a procedure similar to what was done in HMR 52 was developed using the storms influencing the Cherry Creek basin. I believe this is a very good comment and highly support the development of an applications type HMR similar to that information provided in HMR 52 for the entire HMR 55A region. I firmly believe that additional reductions to the 1995 NWS study can be realized but question the magnitude (21 percent) of the likely reduction. Recommend that the A W A team include an explanation as to how they derived the percent reduction of 21 percent. My interpretation of what the A W A team indicates in figure 8.6 of their report would indicate a reduction of 5 to 10 percent. Much additional study needs to be done to determine the appropriate effect of this application to the specific basin let alone a multitude of drainages within the general region. 3. Orographic effects within the basin. The A W A team determined that the PMP from the 1995 NWS study could be additionally reduced by some 9 percent due to the barriers located upwind of the surface inflow. Authors ofHMR 55A evaluated the general terrainlbarriers in the vicinity of the Cherry Creek basin and assuming a NE to SE low level flow from a 72-hour surface low moving through the region south of the Palmer Ridge that minimum orographic rainfall could be produced in the Cherry Creek basin. As seen in figures 1.2 through 1.3 of the A W A report, barriers are minimal, especially in